Wiring a contactor

[Charles LamontParticipant @charleslamont71117]

I am adding a contactor and overload on a single phase machine I am refurbishing. There will be a separate button box.

Is there a preference for putting the control buttons on the live or neutral side of the contactor solenoid?

[Ex contributorParticipant @mgnbuk]

Live side.

Only machines I routinely came across with switching on the neutral side were of Italian origin.

Nigel B.

[noel shelleyParticipant @noelshelley55608]

I’m with Nigel, on the live side. Make sure your contactor has the right coil voltage, ie 220V. Noel.

[EmgeeParticipant @emgee]

Charles

Low cost solution is buy a complete DOL enclosed starter with an enclosure to suit your application, it will have stop/start buttons on the cover. Order with coil voltage to suit your electrical supply and an overload range to provide the flc on the motor nameplate.

Emgee

[David George 1Participant @davidgeorge1]

I have a DOL starter on my lathe and have fitted a reverse switch as well. It is straight forward just follow the diagrams supplied. always switch live side and this set up cuts power if there is a power supply fault and wont start untill the button is pressed after power is recovered.

David

 

 

[Ian PParticipant @ianp]

Switching is generally done on the live side. If the equipment is fed from an isolator then that will most likely switch both poles.

Wiring to buttons on a remote control is best done at low voltage, mains is OK but needs more care and consideration.

Dont forget that if you are using a three phase contactor which has a built in overload, the three sensing coils need to be wired in series for it to trip at the correct settings on the little dial.

Ian P

[SillyOldDufferModerator @sillyoldduffer]

Live only.  Never neutral only, and since about 1950 do not switch both live and neutral.  Read on if interested for the ‘why’.

The UK electrical single-phase system and similar significantly improve safety by ensuring that only one wire (Live / Line) is ‘hot’.   Then it’s not dangerous for users to accidentally contact the Earth Wire or the Neutral.   The risk of electrocution is reduced to touching Live, which will cause a shock to either Earth (Ground) or Neutral.    The purpose of the earth wire was originally to ensure that any metalwork in an installation could not accidentally become live due to loose wires or frayed insulation and electrocute those who touched it.  The earth protection system has since been extended with devices that disconnect Live when an earth current or an imbalance between the live and neutral currents are detected.

As only Live is ‘hot’, it’s only necessary to switch Live off to render the equipment safe.   Switching neutral is dangerous because the although the equipment stops and appears dead, it isn’t.  The internal wiring is still ‘hot’, and many working inside such machines have been zapped as a result.

In the olden days it was considered best practice to switch both live and neutral, belt and braces, but this changed after it was found the live contacts in a dual switch could weld closed whilst the neutral contact stopped and started the machine normally.   This is ‘fail dangerous’ – the operator thinks the machine is safely powered off and it’s still ‘hot’ inside.

When working on electricity it’s best not to trust switches, contactors or people.   Isolate and test before proceeding.  In permanent installations it was once thought sufficient to hang “don’t touch” red-tags on isolating switches in the switch-room.    Soon found that Coco the Clown, perhaps obsessed with getting his job finished, would ignore them, reapply power without asking, and kill somebody.  For that reason modern switches can be padlocked ‘off’, which helps considerably.   Sadly people still get hurt: a few Clowns see no harm in using a bolt-cutter…

Dave

[noel shelleyParticipant @noelshelley55608]

Bearing mind Ians comments, some help can be found in ME, about 2 years ago written by me. Noel.

[Charles LamontParticipant @charleslamont71117]

Thanks all, live side it is, then.

Emgee, I already have the stuff.

David, even with buttons on the neutral side, power loss would drop out the contactor solenoid, and the solenoid circuit would not be restored until the normally open start button was pressed. BTW, I did my Fobco Star a few years ago, after the rotary switch packed up (and repainted it at the same time).

Ian, I will be doing another machine with 24V, but this one is mains control. I do have the correct voltage contactors, and I did know about how to wire a 3-phase contactor for single phase, but thanks for the reminder.

[Maurice TaylorParticipant @mauricetaylor82093]

Hi Dave ,  The live and neutral are both isolated on consumer units.

Double pole isolators are also on cookers etc.

Maurice

 

[Ian PParticipant @ianp]

As you say Maurice, those are isolators

Ian P

[Ian P]

On Ian P Said:

As you say Maurice, those are isolators

Ian P

And as I said, a bit untrustworthy, so after turning off at an isolator, check that the live really is off!

My house is tapped off a three-phase plus neutral circuit strung overhead between poles.   Few years ago, men arrived with a cherry picker to fix something at the end of the street.   I watched one of them check the overhead wires were dead with an earthed hook on the end of a long pole.  Much to everyone’s surprise, the first one tested was still live.

I suspect the incident taught that team more about H&S in a few seconds that all previous courses combined. And I learned that wires on poles have fuses – big ones!   No idea how many amps, but 20 homes at 100A suggests 2000A.   Whatever, one went bang louder than anything I’ve heard at a professional firework display and brought all the neighbours running out to see the damage!  There wasn’t any, though I expect the chap who was supposed to have isolated that section was roasted on a spit later!

Picking up on my post not covering isolators, there’s a lot else I didn’t mention!   I guess a book covering the whys and wherefores of the UK’s different domestic supply arrangements would run to hundreds of pages.  Got a book on Transformers, about 1200 pages of terse technical detail, that doesn’t explain how transformers should be connected: that must be in another hefty volume!

I’ve never seen an explanation of the logic behind why isolators break both lines.  Might be to protect against earth faults, some of which can make neutral hot, or maybe it’s because isolators are rarely operated, making it much less likely that their live contacts will wear and weld together?  Beware! I’m guessing wildly, and don’t know.   The rules are as Maurice says, but someone thought the rules through, and I can’t see their reasoning.  Does Maurice or anyone else know why isolators are different?

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Re isolators. A safety rated mains isolator switch has forced break contacts. This means that the operating knob or lever cannot be physically moved to the off position unless the contacts have moved to the open position. The neutral is switched to protect against voltages that can appear on the neutral due to faults in the network. It also simplifies some fault finding if the neutral – earth connection is broken.

Robert.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Re isolators. …

Robert.

Many thanks, I’ve learned summat! Makes sense.

Never occurred to me that an isolator would do anything different with the contacts.  Clever stuff.

Much appreciated,
Dave

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